Radiation therapy is an established method in which ionizing radiation is used to treat pathological tissue, such as tumor issue for instance. The aim of radiation therapy is to irradiate the tissue to be treated with an adequate therapeutic dose and in the process simultaneously preserve healthy, surrounding tissue. The therapeutic effect is based inter alia on ionizing radiation acting differently on healthy tissue and on pathological tissue.
To ensure that uncertainties in the positioning of the tissue to be treated, which may occur between a planning phase and a treatment phase for different reasons, do not endanger the success of the treatment, safety margins are usually used in order to enlarge the target volume.
Image guided radiation therapy (IGRT) enables uncertainties in the irradiation of the target volume to be reduced. IGRT allows visualization of the target volume, organs at risk (OAR) and healthy, surrounding tissue prior to starting an irradiation so as in principle to enable the target volume to be irradiated more precisely and smaller safety margins to be used.
In order to be better able to determine patient changes between the fractions, a computed tomography recording (also CT for computed tomography) can be produced prior to each irradiation, ideally on the same couch. The position of the tumor can therefore be readjusted.
In order to observe the patient movement during the irradiation, the MV therapy beam can also be used for projection imaging, e.g. for so-called “portal imaging”. Furthermore, systems also exist which in addition to the MV therapy beam, also have a further x-ray source and an additional x-ray-sensitive detector. Aside from projective monitoring of the patient movement, a CT imaging can therefore also be operated as a positioning control.
Sectional images are required in order to create irradiation plans for the radiation therapy, said sectional images representing the region to be irradiated in a three-dimensional fashion. To this end, CT images are predominantly used.